Crystals are orderly arrangements of pure substances. For example,
diamonds are pure carbon, and quartz is pure silicon dioxide. Crystals
form as their component atoms move closer and closer together. This
can occur as pressure and temperature decrease, or as a solvent evaporates.
Once the atoms begin to associate, their geometry and chemical activity
determine the shape of the crystals that form. In general, the size
of the crystals will depend on the amount of substance present in a given
space, and how quickly the atoms can associate. The slower the association,
the larger the crystal.

The fantastic crystal structures in geodes, or hydrolites, form as water
evaporates from the internal spaces of the stones. However, most
crystals are found in rocks that have a volcanic origin. As the super
hot molten rock - called magma - cools, different substances will crystallize
out. Sometimes very large crystals, weighing up to several tons,
can be made as magma slowly cools below the surface of the Earth.

Crystals also can be formed when land masses move. The crust of
the earth can buckle and fold, raising mountains and creating new faults.
Such stresses create great pressures and temperatures that can change the
crystal structures in rocks. These rocks are called metamorphic rocks.
Crystals in metamorphic rocks are generally pretty small.